1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly a system in which an air/fuel ratio of mixture to be supplied to the engine is controlled toward a target value in response to an output signal level of an oxygen concentration sensor.
2. Description of Background Information
Air/fuel ratio feedback control systems for an internal combustion engine are known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and an air/fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and improvements of the fuel economy. As an example of the air/fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533 in which an open-close valve is disposed in an air intake side secondary air supply passage leading to a carburetor of the engine and a duty ratio of the open and close of the open-close valve, i.e. the supply of the air intake side secondary air, is feedback controlled in response to the output signal level of the O.sub.2 sensor.
In the usual air/fuel ratio feedback control systems, it is general that the air/fuel ratio of the mixture supplied to the engine is controlled toward a target air/fuel ratio which is determined previously. Therefore, it is desirable that the amount of the secondary air is controlled by a duty ratio control operation so that the response characteristic of the air/fuel ratio control is fast enough, without causing a large deviation or hunting of the air/fuel ratio from the target air/fuel ratio.